The invention relates to a method for the production of a white light with light emitting diode (=LED) emitting a predetermined color temperature, in the following designated as “white LED”, in which an LED emitting blue light, designated in the following as “blue LED”, or an LED emitting UV light, designated in the following as “UV LED”, is coated with a conversion layer which absorbs the blue light or UV light and emits light of longer wavelength. In the case of a blue LED, a part of the blue emission is converted into a different wavelength region, so that the sum of the emitted light components yields white light. In the case of a UV LED, the conversion layer must naturally contain at least two different color conversion pigments, the emissions of which together yield white light. The invention relates further to a white LED light source, which has a plurality of blue LEDs or UV LEDs, over which a conversion layer is applied.
The color conversion layer is typically of inorganic or organic color conversion pigments, which are distributed in an organic plastic layer.
With regard to a reproducible production of white LEDs within tight tolerances of the resulting chromaticity coordinate, in the production process, along with the concentration of the pigments, also the layer thickness (and its shape) must be exactly defined.
In the state of the art, in this regard various processes are known;    1) The color conversion medium fills the reflector cup, in which the LED is placed, known from (JP 8335719 A-, , JP 8335720 A-. );    2) The color conversion medium is applied in drop form over the LED chip (with transparent spacer:    U.S. Pat. No. 5,959,316 A-, , JP 10190065 A-. );    3) The color conversion medium is applied in the adhesion layer between the LED and a lens( U.S. Pat. No. 6,590,235 B-, );    4) The color conversion medium is definedly produced in layer form and attached to the LED ( EP 1081771 A-, );    5) The color conversion layer is directly printed on the LED chip ( JP 2001244507 A-, ).
These methods meet the requirements for many applications, in particular for illumination, inadequately. The production methods and their tolerances are substantial reasons for the deviations. A further reason is the following:
The excitability of the color conversion agent depends, typically, strongly on the wavelength of the exciting LEDs. Particularly in the case of long wavelength blue LEDs, a wavelength tolerance of only a few nm already leads to significantly different emission intensities with the current color conversion agents. In the case of UV LEDs this would lead primarily to a variation of intensity, in the case of blue LEDs beyond this to a color displacement, since in this case the white light is composed of the blue LED emission and the emission of the color conversion agent.
Since LEDs in accordance with the state of production technology have a relatively broad wavelength distribution, a typical selection class has a wavelength tolerance of more than 5 nm. Even in the case of a very defined application of the color conversion agent (with negligible tolerance) there results from this a tolerance of the color temperature of white LEDs of more than ±300 K. This is already too great for many applications. It thus corresponds to the state of the art, that the market requirement for more narrowly specified white LEDs is satisfied by means of selection processes.